Add constant-heading localizer reset button

Change-Id: I25afc8985dbce2199f6fc4e7eb91c58b1e9bbc5d
diff --git a/frc971/control_loops/drivetrain/localizer.q b/frc971/control_loops/drivetrain/localizer.q
index 8fef686..b1169a9 100644
--- a/frc971/control_loops/drivetrain/localizer.q
+++ b/frc971/control_loops/drivetrain/localizer.q
@@ -7,6 +7,7 @@
   float y;      // Y position, meters
   float theta;  // heading, radians
   double theta_uncertainty; // Uncertainty in theta.
+  bool keep_current_theta; // Whether to keep the current theta value.
 };
 
 queue LocalizerControl localizer_control;